Texas Earthquakes Maybe Triggered by Mining Process

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The injection deep into the earth of wastewater from natural gas
mining may have spurred a series of small earthquakes in Texas,
according to a new study.

Between Oct. 30, 2008, and May 31, 2009, more than 180 minor
tremors were recorded near the Dallas-Fort Worth airport, an area
unaccustomed to shaky ground. The quaking commenced shortly after
the opening of a local disposal well for the liquid byproducts of
natural gas mining — mostly brine, or saltwater.

"When these earthquakes first occurred we had no idea if they
were tectonic or if they
were human-induced," said Cliff Frohlich, a geophysicist at
The University of Texas at Austin.

Natural earthquakes had been felt in other parts of the state,
and human meddling had been tied to such movement elsewhere. The
ground shook shortly after the filling of Lake Mead in the 1930s,
for example, as well as after major
oil extractions and fluid injections around the world.

While science can't prove that the injections triggered the Texas
shaking, Frohlich suggested that new data, which show a large
number of quakes striking close in time and space to the well's
activity, does make a mere coincidence unlikely.

First rumblings

Soon after people in Dallas and Fort Worth were rattled from the
first few rumblings, Frohlich and his team set up a temporary
array of
seismographs. They recorded 11 earthquakes between Nov. 9,
2008, and Jan. 2, 2009, all small enough to go unnoticed by area
residents.

In the study, recently published in the Bulletin of the
Seismological Society of America, the researchers also reviewed
records from permanent seismic-recording stations in Oklahoma and
Dallas for both the earthquakes that were felt and the events
caught by their array.

By all accounts, the origin of the earthquakes appeared to trace
back to the vicinity of the disposal well.

Air hockey hypothesis

But if there were a relationship, Frohlich wondered, then why
would it occur in some spots and not others? There are tens of
thousands of other places where people have pumped fluids into
the earth without sending the ground rolling.

His best guess: the 2.6-mile (4.2-kilometer) deep well sits
"close to a favorably oriented fault," he said.

Frohlich pointed to what he calls the "air hockey hypothesis." In
the game, a slight push sends a plastic puck sliding across the
table. But if the air under the puck is not turned on, friction
keeps it in place.

"Similarly, if you pump fluids into the earth where there is a
fault, it might push the sides of the fault apart just enough so
that the rock can slip," he explained.

In Dallas-Fort Worth, the slips were small: no quake exceeded a
magnitude 3.3. "We're not expecting large earthquakes here,"
Frohlich said.

Reinjection

Frohlich also emphasized that drilling doesn't appear to be the
problem. There are four separate processes in the production of
natural gas. After the initial drilling into gas-rich rock,
pressurized water loaded with sand is injected to crack the
formation and release the gas, a step known as fracking. The
resource is then pulled to the surface, along with some unwanted
fluids that are later separated from the natural gas and pumped
back into deep rock at a nearby site.

It's only this last "reinjection" step, said Frohlich, which is
"possibly" to blame.

"That's good news if you are a producer," he added. "If you think
you're near a fault, you can take the fluids somewhere else."

But Chesapeake Energy, which runs the natural gas operations in
the Dallas-Fort Worth region, doesn't plan to truck brine afar
anytime soon. They remain unconvinced that their work is related
to the seismic activity. "If saltwater disposal is the cause of
the seismicity, it is very difficult to explain why the
seismicity has continued for well over a year after the well was
shut-in," Julie Wilson, vice president of urban development for
the corporation, told OurAmazingPlanet.

Chesapeake Energy's disposal operations stopped in August 2009,
she added, while sequences of similar earthquakes were recorded
in May, June and November 2010.

One possible explanation for delayed shaking is that it could
take a long time for the injected water to travel, and for
pressures and stresses to equilibrate, Frohlich said. A slew of
earthquakes in Denver in the early 1960s, blamed on injections of
wastewater by the U.S. Army, were similarly drawn out, he said.

As in Texas, none of the Denver events exceeded magnitudes
previously recorded in the area. Larger natural earthquakes hit
Colorado, however, and the induced series did include a magnitude
5.3 quake. Despite these potential negative consequences,
Frohlich still sees an important place for natural gas production
in the United States.

"Unless you're going to turn off your air conditioner, you need
something that provides a bridge for the next 20 to 30 years,"
said Frohlich, alluding to the transition toward alternative
energies. "Natural gas is about as clean a fuel as any."